The present invention relates to motion detectors and in particular, to a combination passive infrared sensor and microwave sensor with anti-masking testing logic.
Dual technology motion detectors, and in particular, a passive infrared sensor, in combination with a microwave sensor, has certain desirable characteristics. The passive infrared sensor detects a moving heat source, whereas the microwave sensor transmits microwave signals and evaluates returned signals for a moving object. By combining these two technologies, the detector is less sensitive to false alarms. Normally, the detector only produces an alarm signal if both sensors detect motion.
Masking is a technique where the detector is partially covered or shielded such that the detector does not monitor the actual space. The passive infrared sensor can be coated with a paint or other non transparent coating and this coating acts as a barrier between the sensor and the monitored space. In most cases, these coverings are essentially transparent to the microwave signals. It is also possible to mask the entire detector by placing a box around the detector. In this case, both the microwave sensor and the passive infrared sensor are effectively masked.
Some detectors include a feature to predict whether a masking event has occurred. Unfortunately, these techniques identify situations which are typical of masking but not limited to masking. For example, the detector may determine that a person has come in close proximity to the detector and on this basis, predicts that a masking event has occurred. In actual fact, the person may have inspected the sensor for perfectly valid reason and subsequently departed. This approach for identifying possible masking events produces false alarms.
The present invention provides a detector with improved characteristics with respect to detecting possible masking conditions.
A motion detector according to the present invention, comprises a passive infrared sensor and a microwave sensor in combination with a processing arrangement for processing the signals from the sensors and determining alarm conditions. When an alarm condition is determined, the detector produces an alarm signal. The processing arrangement includes logic for recognizing a condition which indicates a possible masking opportunity and upon recognizing a masking opportunity, the logic initiates an anti-masking process. The anti-masking process reviews the signals of the sensors after the initiation of the anti-masking process and compares the signals to a predetermined series of possible sensor signals. This predetermined series of possible sensor signals includes some of which indicate normal non alarm conditions and others which represent alarm conditions. The logic produces an anti-masking alarm when the comparison of the signals indicates an alarm condition.
According to an aspect of the invention, the anti-masking process reviews the signals from both sensors for a predetermined period of time following detection of a masking opportunity.
According to yet a further aspect of the invention, the sensor signals which indicate non alarm conditions include the microwave sensor producing signals indicating a person moving away from the sensor and the passive infrared sensor continues to sense a person.
According to yet a further aspect of the invention, the sensor signals which indicate an alarm condition include 1) said microwave sensor produces signals indicating a person is moving away from the sensor and the passive infrared sensor fails to detect a person and 2) the microwave sensor fails to detect a person and the passive infrared sensor fails to detect the person.
According to yet a further aspect of the invention, the microwave detector is downwardly angled approximately 30 degrees to allow viewing of the space below the sensor.
According to yet a further aspect of the invention, the microwave sensor includes a self test feature to evaluate the operational status of the microwave sensor.
According to yet a further aspect of the invention, the processing arrangement includes a table of possible pairs of sensor readings and upon initiating an anti-masking process, compares the signals to said sensors after a predetermined time period with said table of possible pairs of sensor readings, and based thereon, makes a determination whether the anti-masking alarm signal is produced.